1. Technical Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit structured such that two electrode assemblies are disposed opposite to each other in a central sealed chamber in which light emitting substances are enclosed. Each electrode assembly includes an electrode rod, molybdenum foil and a lead wire, and is sealed in a pinch seal portion adjacent to the central sealed chamber. Each pinch seal portion includes a residual-compressive-stress layer. The present invention also relates to a method for manufacturing an arc tube with a residual-compressive-stress layer.
2. Prior Art
FIG. 6 shows a conventional discharge lamp unit that incorporates an arc tube 5 having a front end supported by one lead support 2 projecting forward from an insulating base 1. A recess 1a of the base 1 supports the rear end of the arc tube 5. A metal support member S, secured to the front surface of the insulating base 1 holds a portion of the arc tube adjacent to the rear end of the arc tube. A front lead wire 8, extending from the arc tube 5, is welded to the lead support 2, while a rear lead wire 8 penetrates a bottom wall 1b having the recess 1a of the base 1 formed therein. Then, the rear lead wire 8 is, by welding, secured to a terminal 3 provided for the bottom wall 1b. Symbol G represents an ultraviolet-ray shielding globe arranged to remove an ultraviolet-ray component in the wavelength region harmful to the human body. The ultraviolet-ray shielding globe forms a cylindrical shape and is integrally welded to the arc tube 5.
The arc tube 5 has a sealed chamber portion 5a formed between a pair of front and rear pinch seal portions 5b. The sealed chamber portion 5a has electrode rods 6 disposed opposite to each other and contains light emitting substances. In the pinch seal portions 5b, the sealed molybdenum foil 7 connects the electrode rod 6 projecting into the sealed chamber portion 5a to the lead wire 8 extending from the pinch seal portion 5b. Thus, the pinch seal portions 5b remain airtight.
Preferably, the electrode rod 6 is made of tungsten exhibiting excellent durability. Tungsten has a coefficient of linear expansion that is considerably different from that of the quartz glass that constitutes the arc tube. Worse, only unsatisfactory conformability with quartz glass is permitted and the permitted airtightness is unsatisfactory. Therefore, the molybdenum foil 7 having a coefficient of linear expansion similar to that of quartz glass and exhibiting relatively satisfactory conformability is connected to the tungsten electrode rods 6. Moreover, the pinch seal portion 5b seals the molybdenum foil 7. Thus, pinch seal portions 5b remain airtight.
Referring to FIG. 7(a), a method of manufacturing the arc tube 5 is illustrated. An electrode assembly A comprises an electrode rod 6, molybdenum foil 7 and a lead wire 8. The components are integrally connected. The electrode assembly A is initially inserted into an end of either opening of a cylindrical glass tube W having a spherical expanded portion w2 disposed at an intermediate position of a straight extending portion w1. Then, adjacent position q1 of the spherical expanded portion w2 undergoes a primary pinch-seal operation.
Referring to FIG. 7(b), a light emitting substance P and the like are introduced into a spherical expanded portion w2 through the other end opening of cylindrical glass tube W. Referring to FIG. 7(c), a second electrode assembly A is inserted. A secondary pinch sealing operation seals the spherical expanded portion w2, while simultaneously cooling the spherical expanded portion w2 by using liquid nitrogen to prevent both vaporization of the light emitting substance P and heating the adjacent position q2 of the spherical expanded portion w2. The final result is an arc tube 5 having the chipless sealed chamber portion 5a. 
Referring to FIG. 7(b), the primary pinch-sealing operation uses inactive gas (in general, which is low-cost argon gas or nitrogen gas) as forming gas into the glass tube W in order to prevent oxidation of the electrode assembly A. Referring to FIG. 7(c), in the secondary pinch-sealing operation, the ends of the openings in cylindrical glass tube W are closed and cooling with liquid nitrogen prevents vaporization of the light emitting substance P. Therefore, a state of near vacuum is necessary for the pinch-sealing operation.
Since a large temperature change occurs between a state where the arc tube is turned on and a state where the arc tube is turned off, thermal stress occurs between the electrode rod and the glass layer. The electrode rod and the glass layer each have considerably different coefficients of linear expansion when the arc tube is turned on. In recent years, the arc tube structure now lights instantaneously. Therefore, a high temperature-rise ratio is realized. After repeated cycling, a crack forms in the pinch seal portion (the glass layer) for sealing the electrode rods 6. Thus, the sealed substances leak, thereby causing a defect in the lighting of the arc tube and shortening its life.
In view of the foregoing, the inventor has repeatedly performed experiments and studies to solve the foregoing problems experienced with the conventional technique. As a result, the inventor discovered that retention of compressive stress produced in the pinch seal portions 5b during the arc tube manufacturing process causes a thermal stress in the glass layer in the pinch seal portion to disperse due to rise in the temperature occurring after turning the arc tube on. Therefore, prevention of the formation of a crack in the glass layer in the pinch seal portion will extend the life of the arc tube.
The present invention solves the problems experienced with the conventional technique and in accordance with the inventor""s discovery. An object of the invention is to provide an arc tube for a discharge lamp unit that is free of crack formation in the pinch seal portion when the thermal stress changes due to arc tube cycling.
To achieve the object, an arc tube for a discharge lamp unit comprising at least two electrode assemblies, each of the electrode assemblies comprising an electrode rod, a foil and a lead wire integrally connected in series, a tube having a central sealed chamber enclosing light emitting substances, and further comprising pinch seal portions disposed at opposite ends of the chamber, each pinch seal portion enclosing an electrode assembly such that the electrode rod projects into the chamber and the lead wire projects from the pinch seal portion, and a residual-compressive-stress layer facing a glass layer region in each of the pinch seal portions, the residual-compressive-stress layer hermetically contacting the electrode rod, wherein the residual-compressive-stress layer and the glass layer region extending only along the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed for a length greater than or equal to 30% of the axial length of the glass layer region that only contacts the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed in an angular range of about 180xc2x0 or larger in the circumferential direction of the electrode rod.
According to another aspect of the invention, the residual-compressive-stress layer is formed for a length greater than or equal to 30% of the axial length of the glass layer region that only contacts the electrode rod and in an angular range of about 180xc2x0 or larger in the circumferential direction of the electrode rod.
No thermal stress is produced in the boundary between the glass layer and the electrode rod immediately after the pinch-sealing operation. When the temperature returns to room temperature, the boundary between the electrode rod (made of tungsten) and the glass (quartz glass) encounters generation of thermal stress (tensile stress in the electrode rod and compressive stress in the glass layer). The thermal stress corresponds to the difference between the coefficient of linear expansion of the electrode rod and that of the quartz glass. Therefore, a state in which great stress (the tensile stress in the electrode rod and the compressive stress in the glass layer) is produced is maintained.
After lamp turn on, the arc tube temperature does not rise to a level at which the pinch seal portion is pinch-sealed. Therefore, when the residual-compressive-stress layer on the glass layer has been formed over a wide range, the thermal stress produced in the glass layer of the arc tube after lamp turn-on causes the compressive stress left in the glass layer of the pinch seal portion to be reduced in both of the axial direction and the circumferential direction.
That is, the thermal stress (tensile thermal stress) for relaxing the residual compressive stress acts on the glass layer in the pinch seal portion when the lamp is turned on. When the residual-compressive-stress layer is too small, the thermal stress is concentrated to the small residual-compressive-stress layer. When the lamp is repeatedly turned on and off, the thermal stress is repeatedly acts upon the glass layer. Thus, there is a possibility that a crack allowing the sealed light emitting substances to leak can form. Specifically, when the axial length of the residual-compressive-stress layer is shorter than 30% of the axial length of the glass layer region which hermetically contacts only the electrode rod, the thermal stress in the axial direction cannot sufficiently be absorbed. In the foregoing case, concentration of the stress to the residual-compressive-stress layer causes the sealed light emitting substances to leak through the glass layer. When the angular range of the residual-compressive-stress layer in the circumferential direction of the electrode rod is smaller than about 180xc2x0, the thermal stress in the circumferential direction cannot sufficiently be absorbed. Thus, the stress is concentrated to the residual-compressive-stress layer and, therefore, a vertical crack of the glass layer forms that allows the sealed light emitting substances to leak.
The compressive stress layer is previously formed in a predetermined wide region in the axial direction or/and the circumferential direction on the surface of hermetic contact between the glass layer and the electrode rod. Therefore, the compressive stress layer (the residual compressive stress layer) formed in the large range efficiently relaxes (absorbs) the thermal stress produced in the glass layer as the temperature is raised.
Namely, the residual-compressive-stress layer present over a predetermined large range disperses the thermal stress that is repeatedly produced before the thermal stress is transmitted to the glass layer. Therefore, the glass layer does not crack and none of the sealed substances leak.
According to another aspect of the invention, the residual-compressive-stress layer has a boundary crack formed in the outer surface of the residual-compressive-stress layer.
The thermal stress acting on the boundary between the electrode rod and the glass layer after the lamp has been turned on is absorbed because the glass layer slides along the boundary crack.
According to another aspect of the invention, the pinch seal portion in which the electrode rod is sealed is pinch-sealed in a temperature range from 2000xc2x0 C. to 2300xc2x0 C., preferably in a temperature range of 2100xc2x0 C. to 2200xc2x0 C.
Quartz glass has a softening point of 1600xc2x0 C. Moreover, the permissible machining temperature is 1800xc2x0 C. Therefore, when the temperature of the glass tube (a portion which must be pinch-sealed) is 2000xc2x0 C. or lower, the temperature in the glass layer (a portion including the electrode rod) is not raised to a level which is sufficiently high to maintain the adhesion with the electrode rod. Preferably, to form the residual-compressive-stress layer in a large area in the axial direction and the circumferential direction of the electrode rod, the pinch seal portion (in which the electrode rod is sealed) is pinch-sealed at a temperature of 2000xc2x0 C. or higher, more preferably 2100xc2x0 C. or higher.
When the temperature of the glass tube (the portion which must be pinch-sealed is 2300xc2x0 C. or higher, no effect to enlarge the residual-compressive-stress layer can be obtained. Moreover, the pincher for pinch-sealing the glass tube and the arc-tube support member must exhibit severe heat resistance during the pinch-sealing operation. Preferably, to efficiently form the residual-compressive-stress layer, the pinch seal portion (in which the electrode rod is sealed) is pinch-sealed at a temperature of 2300xc2x0 C. or lower, preferably 2200xc2x0 C. or lower.